Time of flight (TOF) mass spectrometers have developed into well established analytical instruments for identifying materials based on a distribution (spectrum) of charged particles differing in mass created by pulsed radiant energy or particle bombardment. A sample of material whose spectrum is sought is mounted as a target in an electric field. Bombardment with accelerated particles, such as perfect gas atoms or ions, or high intensity electromagnetic radiation, disrupts the molecules of the target to create a variety of charged particles--e.g., molecular ions, fragments, cations, and/or anions--hereinafter collectively referred to as ions. Once an ion of the sample material is created, it is accelerated in the electric field toward an electrode of opposite charge. A portion of accelerated ions is allowed to pass through an aperture in the attracting electrode and embark on a flight path which, through creation of an ambient vacuum, can be of extended length.
When the target sample receives a bombardment pulse, parcels of ions of like polarity but differing in mass are generated. Given that each ion creating collision imparts the same momentum EQU mv
where
m is mass and PA1 v is velocity, PA1 d is distance and PA1 t is time,
it follows that ions of greater mass have a lower velocity. Since velocity is EQU d/t
where
it follows that ions differing in mass within any single parcel will arrive at different times at a reference location along their common flight path. Stated another way, the original parcel of ions created by the bombardment pulse divides itself into partial parcels consisting of ions of the same mass and differing in mass from the ions of other partial parcels. By measuring and comparing the time of flight of partial parcels a spectrum of flight times can be identified which can then be mathematically translated into a mass spectrum unique to the sample material.
If all the ions in each partial parcel entered the flight path with exactly the same initial energy, then very compact (highly focused) partial parcels each consisting of ions of identical mass would be created. In practice there is a range of kinetic energies initially imparted to the ions within a partial parcel and this can lead to a range of flight times of ions within any given partial parcel that is broad enough to overlap flight time ranges of adjacent partial parcels.
The solution to this problem has been to provide a focusing deflection field in the flight path. The deflection field causes the partial parcels to traverse one or more arcs. In so doing, within each partial parcel the ions of higher kinetic energies in undergoing the same angular deflection traverse arcs of longer radii than ions of lower kinetic energies. Thus, the time required for ions of differing kinetic energies within each partial parcel to traverse the deflection field is evened out by the unequal arc paths. By locating the deflection field between time measurement reference locations in the flight path, usually referred to as entrance and exit planes, the result is to focus the partial parcels. Stated another way, the function of the deflection field is to make the flight time of ions in each partial parcel a function of the ratio of ion mass (m) to charge (e) rather than initial differences in kinetic energies.
As has been mathematically demonstrated to the satisfaction of those skilled in the art, quadruple focusing (four deflection arcs) are required to bring the partial parcels of ions exiting the deflection field into focus spatially (as measured along the three mutually perpendicular axes of space, usually referred to as X, Y, and Z axes), as well as in terms of elapsed time of flight (t), momentum (mv), and kinetic energy (0.5mv.sup.2). In order to achieve focusing of the ions leaving the deflection field it is further necessary that the deflection arcs be chosen so that they are symmetrical with respect to a central point on the ion flight path within the deflection field.
A schematic diagram of a conventional quadruple focusing time of flight (QFTOF) mass spectrometer containing a deflection field is shown in FIG. 1. The mass spectrometer 100 is comprised of a central vacuum chamber 102 defining an ion flight path indicated by arrows 104 extending between an entrance plane 106 and an exit plane 108. The ambient pressure in the vacuum chamber is maintained below 1.33.times.10.sup.-4 kilopascals (&lt;10.sup.-5 torr) to minimize ion collisions with the ambient atmosphere. There is located in the vacuum chamber between the dashed lines 110 and 112 a deflection zone 114. A pulsed ion source 116 emits a parcel of accelerated ions across the entrance plane into the flight path within the vacuum chamber. The ion source is also internally evacuated and can therefore be viewed as an extension of the flight path vacuum chamber. Beyond the exit plane there is located a receiving unit 118 for the ions traveling along the flight path. The receiving unit forms a second extension of the ion flight path vacuum chamber. By referencing the time at which receipt of a partial parcel is detected to the time a target pulse was generated in the ion source, a measurement of the time elapsed in traversing the flight path vacuum chamber between its entrance and exit planes can be provided.
The conventional QFTOF spectrometer shown in FIG. 1 focuses the partial parcels of ions by directing the flight path through four separate deflection arcs which are arranged to be symmetrical about a central point S in the flight path. Each of the deflection arcs lies in a common central reference plane with limited divergence of ions from the central reference plane being permitted.